1) Field of the Invention
This invention relates to an improvement in an air bag system which is mounted on an automotive vehicle or the like and is inflated in the event of a vehicular collision to protect an occupant from impact, and more specifically to a connection structure for an air bag cover and a housing supporting a module thereon and mounted on a vehicle body in an air bag system whereby the strength of connection between the air bag cover and the housing can be enhanced.
2) Description of the Related Art
As a cushioning system for providing additional safety beyond that provided by a seat belt system for holding safely an occupant or the like of an automotive vehicle in a seat, an air bag system has been put into practical use in recent years. With the air bag system, the body of an occupant, especially his head and chest, is supported by a balloon-like air bag in the event of a collision and the internal gas is then gradually discharged, whereby a cushioning stroke is prolonged to reduce impact to the body.
An air bag system of the type mentioned above includes, as is shown in FIG. 14, a module composed of a folded air bag 1, an inflator (gas generator) 2 and the like. The module is accommodated in an air bag cover 3 which can be ruptured open in the event of an emergency. Upon receipt of an electric signal from a sensor which has detected a vehicular collision, the inflator 2 is ignited so that the air bag 1 is instantaneously inflated by the resulting gas. The air bag cover (hereinafter simply called the "cover") 3 has a structure such that, when the air bag system is actuated, the air bag cover is ruptured open by the expanding pressure of the air bag 1 to permit substantial inflation of the air bag 1 through the thus-ruptured opening over the cover 3.
Such an air bag system is mounted on a steering wheel or the like generally through such assembly steps as will be described next. To assemble the air bag system, the air bag 1 is attached to a housing 4 by fastening means such as rivets. The air bag cover 3 is then fitted on the housing 4, whereby the air bag 1 is covered by the air bag cover 3. At this time, while slightly widening a peripheral side wall 3a of the cover 3, hook portions 3b formed at a free edge of the peripheral side wall 3a and bent inwardly of the cover are brought into engagement with a free edge 49 of the housing 4 so that the cover is temporarily secured on the housing. Fastening means 7 such as rivets are then inserted from the outside of the cover through insertion holes into the interior of the housing via protective strips 6. Free end portions of the rivets, said free end portions extending from an inner wall of the housing 4, are subjected to plastic deformation, whereby the cover 3 is firmly secured on the housing 4.
The inflator 2 is thereafter mounted on the housing 4 by an unillustrated mounting means to form a module. The module is then mounted on a steering wheel.
In the air bag system assembled as described above, the hook portions 3b function as temporary securing means as described above. In addition to this function, the hook portions 3b are also provided to withstand opening force applied to the cover upon expansion of the air bag so that the cover 3 is prevented from being separated from the housing 4 by the opening force.
Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 1-109146, which corresponds to U.S. Pat. No. 4,989,897, discloses another air bag cover with projections (3,18,19) instead of the above-mentioned hook portions 3b, said projections extending outwardly from the air bag cover or both outwardly and inwardly from the air bag cover so that the cover 3 is prevented from being separated. As has been described above, the connection between a cover and a housing in conventional art is effected by bringing hook portions, which are provided along an edge portion of an opening of the cover, into engagement with the free end edge of the housing and holding a peripheral side wall of the cover between a protective strip and the housing, whereby the cover is firmly secured on the housing.
In the conventional structures described above, the opening force applied to the cover upon expansion of the air bag is, however, first exerted on the fastening means, such as rivets, inserted through the cover so that high shear load is applied around the rivet-inserted holes and is then transmitted to the hook portions located on a side rear of the fastening means relative to the opening force. Accordingly, the hook portions do not have substantial resistance to the opening force and contribute only after deformation of the rivet-inserted holes. Nevertheless, the formation of the cover is complicated due to the need for the formation of the hook portions having such a little function. Further, the formation of the hook portions has led to an increase in the dimensions of the cover. This is certainly not preferred from the standpoint of external appearance and also where the mounting space for the air bag system is extremely limited. In addition, the temporary securement of the cover is time-consuming where the hook portions are provided along the entire periphery of the edge portion of the opening of the cover.